Fabry-Perot optical resonant cavity systems are well known, going back to the 19th Century Fabry-Perot interferometer or etalon in which two parallel partially silvered plane glass plates spaced apart at a fixed distance produce interference spectra by multiple reflection therebetween.
Fabry-Perot optical resonant cavity systems are known to be useful in spectroscopy, linewidth reduction in lasers, optical spectral analysis, optical communication, wavelength division multiplexing, multichannel and tunable spectral filtering, and spectral noise reduction.
In term of spectra, three categories of Fabry-Perot interferometers can be recognized: flat-mirror cavities, confocal and non-confocal cavities. Flat-mirror cavities have clear and unambiguous spectral lineshape, i.e. they have no visible transverse modes. However, they have the disadvantage that waveform diffraction ,acts to limit the overall cavity finesse and efficiency, reducing its resolution and use, such as in communication multi-channel filtering. Confocal cavities also have clear and unambiguous lineshape, however, they cannot be manufactured with large Free Spectral Ranges due to the severe requirement for short radii of curvatures of the mirrors. Also, the radius necessary for confocality restricts the optical path length achievable with confocal mirror cavities.
Non-confocal cavities can have large Free Spectral Ranges and super high finesse, and overcome waveform diffraction by continuously refocussing the resonant waveform inside the cavity. However, non-confocal cavities suffer from the presence of non-degenerate transverse modes that limit an unambiguous spectral filtering capability of the device. That is also the case with flat-mirror or other cavities that have lens between the mirrors which produces non-degenerate transverse modes that limit an unambiguous spectral filtering capability of the device, and that therefore may be classified as non-confocal cavities, in contradistinction to pure flat-mirror cavities of the above mentioned kind.